Cycles. Box on font line unable to delete
After you converted the text to a mesh(That's how it was when i downloaded the .blend), there was more than one, many in fact, squares overlapping it's self.

Just select all, press W and select remove doubles. Then, select the square again and delete it.

A step by step guide:


Select the text.
Enter Edit mode.
Select all and then press W then press Remove doubles.
Press Z to go into Wireframe mode. 
Press C to turn on Circle select.
Select the square using Circle select.
Press X or Delete and then press Vertices .
Exit Edit mode, and it should be done. :)


Link to the process.